Diisocyanates are produced in large volumes and are mainly used as starting materials to produce polyurethanes. Their preparation usually takes the form of the corresponding diamines being reacted with phosgene. One possible way to prepare diisocyanates is to react the diamines with the phosgene in the gas phase. This form of the reaction, which is typically known as a gas phase phosgenation process, requires the reaction conditions to be chosen such that at least the reaction components diamine, diisocyanate and phosgene, but preferably all the starting materials, assistants, and intermediate and final products of the reaction are gaseous at the chosen conditions. Advantages of gas phase phosgenation include a reduced phosgene hold-up, less by-production, simpler working up to the desired diisocyanate and increased yields for the reaction. The present invention relates solely to gas phase phosgenation.
Various processes for preparing isocyanates by reaction of amines with phosgene in the gas phase are known from the prior art.
EP 0 593 334 B1 discloses a process for preparing aromatic diisocyanates in the gas phase by using a tubular reactor. The walls are constricted to mix the reactants therein. The reaction takes place in the temperature range from 250° C. to 500° C. Yet said process is problematic because reactant stream mixing solely via a narrowing in the tube wall does not work very well by comparison with employing a proper mixing element. Poor mixing typically leads to an undesirably high formation of solids. Ureas for instance may form by dint of a reaction of already formed isocyanate with as yet unconverted starting amine.
There has been no shortage of attempts to minimize this formation of solids, which is especially observable in the reaction of aromatic diamines with phosgene in the gas phase, in order to thereby make possible a gas phase phosgenation of aromatic diamines which is implementable on a large industrial scale. The focus of the improvements in the process for the large scale industrial phosgenation of aromatic diamines in the gas phase is on improving the mixing of the reactant streams and also on homogenizing the flow in the gas phase reactor which lead to a prolonged operating life for the gas phase reactor. In addition to poor mixing of the reactants, an excessive thermal stress on the aromatic diamine also leads to an increased formation of solids, as for instance by ammonia elimination and subsequent formation of ammonium chloride, which specifically at the recovery stage of the phosgene used in excess may come down as a deposit in the corresponding apparatus.
EP 0 570 799 B1 discloses a process for preparing aromatic diisocyanates which is characterized in that the reaction of the related diamine with phosgene is carried out in a tubular reactor above the boiling temperature of the diamine within a mean residence time of 0.5 to 5 seconds, and in which the mean deviation from the mean residence time is less than 6%. Compliance with this contact time is achieved by performing the reaction in a pipe flow regime characterized either by a Reynolds number of above 4000 or a Bodenstein number of above 100. When the pipe flow regime is characterized by a Reynolds number above 4000, the disadvantage is again that, owing to the high rates of fluid velocity needed, the residence time needed for complete conversion of the amines can only be realized in very long mixing and reactor tubes. According to EP 0 570 799 B1, both excessively long and excessively short residence times lead to undesired formation of solids, and therefore the flow in the reaction space has to be homogenized and particularly any backmixing of the components in the reaction space must be foreclosed. EP 0 570 799 B1 discloses that appropriately short mixing times are obtainable using conceptually known methods based on mixing assemblies having moving or static mixing elements, preferably static mixing elements, although, according to EP 0 570 799 B1, it is particularly the application of the jet mixer concept (Chemie-Ing.-Techn. 44 (1972) p. 1055, FIG. 10) which delivers sufficiently short mixing times. EP 0 570 799 B1, however, does not address in any detail the conditions for vaporizing the diamine and the pressure conditions between the vaporization space and the reaction space.
Measures for homogenizing the flow conditions are likewise addressed by EP 1 362 847 B1. EP 1 362 847 B1 discloses a process for preparing aromatic diisocyanates in the gas phase wherein an improved way of performing the reaction in tubular reactors due to fluid-engineering measures such as the homogenization and centerization of the reactant streams serves to avoid temporal variations of the temperature and any asymmetry in the temperature distribution, which are taught by EP 1 362 847 B1 to lead to caking and plugging in the reactor and hence to a shortened operating life for the reactors.
EP 1 449 826 A1 teaches in relation to the reaction of aromatic diamines with phosgene in the gas phase that the reaction of the phosgene with the diamine to form the diisocyanate has to compete against the subsequent reaction of the diisocyanate with the diamine to form the corresponding urea oligomer, and that improved mixing of the starting reactants—phosgene and diamine—coupled with concurrent avoidance of backflow in the tubular reactor enhances the selectivity of diisocyanate formation and reduces urea formation. As a result, so the teaching of EP 1 449 826 A1, it is possible to reduce the amounts of condensation product in the tubular reactor which, owing to their deposition on the reactor wall, lead to a reduction in the clear cross section through the tube and to a gradual increase in the pressure in the reactor and ultimately determine the on-stream time of the process. EP 1 449 826 A1 discloses the use of a so-called multiple nozzle to ensure the improved mixing of the starting reactants, phosgene and diamine. However, EP 1 449 826 A1 does not disclose any details for transferring the diamines into the gaseous state and for the pressure conditions between the vaporization space and the reaction space.
Hardware solutions for improved mixing of the starting reactants are likewise disclosed by EP 1 526 129 A1, DE 103 59 627 A1 and WO 2007/028 715 A1, in that fluid-engineering measures for twist generation (EP 1 526 129 A1 [e]), concentrically disposed annular nozzles with singular (DE 103 59 627 A1 [e]) or multiple amine feeding (WO 2007/028 715 A1) as well as a plurality of amine nozzles disposed parallel to the axis of a tubular reactor (EP 1 449 826 A1) are employed therein.
EP-A-1 526 129 teaches that increased turbulence for the starting reactant stream in the central nozzle has a positive effect on the commixing of said reactants and hence on the gas phase reaction as a whole. The better commixing reduces the proclivity for by-product formation and significantly shortens the required residence time and thus reactor design length.
EP 1 526 129 A1 discloses a shortening of the mixing sector to 42% of its original length on using a spiral helix as turbulency-generating internal fitment in the central nozzle. True, EP 1 526 129 A1 does describe the need for the amine to be vaporized before being sent into the reaction space. Yet since no data whatsoever are disclosed for the corresponding pressure conditions, the notional person skilled in the art is forced to assume that the pressure level is higher in the vaporization space than at the point of entry to the reaction space in order that the desired flow regime may be realized.
But not just the physicotechnical reaction conditions but likewise the properties of the aromatic amines introduced into the reaction with phosgene in the gas phase have been the subject of disclosed processes.
WO 2008/071564 A1 for instance teaches that amines to be converted into the corresponding isocyanates in a gas phase phosgenation have to meet certain requirements. Specifically, the decomposition rate of such amines throughout the duration of the reaction under the prevailing reaction conditions in the gas phase reactor has to be not more than 2 mol %, more preferably not more than 1 mol % and most preferably not more than 0.5 mol %. The aliphatic or cycloaliphatic amines qualify according to WO 2008/071564 A1, but it is also possible to use aromatic amines provided they are transferable into the gas phase without significant decomposition. Directions as to how the aromatic diamines termed suitable can be transferred into the gas phase without significant decomposition or how any decomposition proclivity may at least be reduced, however, are not derivable from WO 2008/071564 A1.
A specific vaporizing technique to limit the thermal stress imposed on the amines used in a gas phase phosgenation is disclosed by EP 1 754 698 A2. EP 1 754 698 A1 teaches that the deposits observed in the reactor for reacting the amines used with phosgene are firstly caused by the amines used decomposing during the reaction. Secondly, long residence times in the vaporization stage and superheating specifically with the use of aliphatic amines lead to the amines used undergoing a partial decomposition by elimination of ammonia. This partial decomposition during vaporization by elimination of ammonia, observed specifically with the use of aliphatic amines, does not just reduce the yield, but in the subsequent phosgenation reaction also leads to deposits of ammonium chloride forming in the downstream pipework and apparatus. The plant equipment then has to be cleaned relatively frequently, giving rise to corresponding production losses. EP 1 754 698 A1 discloses that these disadvantages arise particularly with the tube bundle heat exchangers, plate heat exchangers or falling film vaporizers typically employed for vaporizing and superheating the amines. By way of technical solution, said document discloses preventing the elimination of ammonia during the vaporization by using specific millisized or microsized heat exchangers for vaporizing and superheating aliphatic amines.
One disadvantage with the microsized heat exchangers disclosed in said document EP 1 754 698 A1 is the very small size of their channels, so that even very small amounts of solid material, which are always present in industrial processes, lead to plugging and thereby shorten the on-stream time of the vaporizer. A further disadvantage with the total vaporization disclosed for the amines is that the amines must not contain any nonvaporizable constituents, since these would inevitably form a solid residue on the vaporizer surface, hence impair the heat transfer and finally lead to plugging of the vaporizer. However, providing amines of the required quality in an industrial process is very burdensome and costly. Thus, although the teaching of said document does improve the on-stream time of the reactor, that of the vaporizer is significantly curtailed, so overall the on-stream time of the manufacturing plant is not gainfully improved.
Minimizing the thermal stress for the amines, during their vaporization for reaction with phosgene in the gas phase, is likewise the subject of EP 1 935 876 A1. EP 1 935 876 A1 discloses that the amines, before they are reacted with phosgene, are generally vaporized and heated to 200° C. to 600° C. and, optionally diluted with an inert gas such as N2, He, Ar or with the vapors of an inert solvent, for example aromatic hydrocarbons optionally with halogen substitution, e.g. chlorobenzene or o-dichlorobenzene, are sent to the reaction space. According to said document, the step of vaporizing the starting amines can be effected in any known vaporization apparatus, preference being given to the use of vaporization systems where a small hold-up is circulated at a high rate through a falling film vaporizer while the vaporization process is optionally augmented by injection of inert gas and/or vapors of an inert solvent in order to minimize the thermal stress for the starting amines.
WO 2010/010 135 A1 discloses a process wherein the starting reactants—amine and phosgene—are transferred via an ejector into a mixing zone followed by a diffuser for pressure and temperature increase. This allows vaporization of the amine at a lower pressure than prevailing in the reactor, lowering the boiling temperature of the amine. The disadvantage with this process is that the degree of pressure reduction in the amine vaporizer is greatly dependent on the amine:phosgene ratio and thus on the phosgene excess. A change in the reaction conditions will thus also always have an effect on the pressure in the amine vaporizer and the temperature prevailing there.